1. Field of the Invention
This invention relates to metal halide discharge lamps which operate at power levels below 40 watts and have efficacies exceeding 35 lumens per watt. More particularly this invention relates to an electrode structure, which in combination with the quartz tube geometry and the fill of mercury, metal halide, and noble gas, makes the high efficacy possible and permits prolonged operation at nearly constant voltage levels and lumen output.
2. Description of the Prior Art
Metal halide discharge lamps typically have a quartz tube that forms a bulb or envelope and defines a sealed arc chamber, a pair of electrodes, e.g. an anode and a cathode, which penetrate into the arc chamber inside the envelope, and a suitable amount of mercury and one or more metal halide salts, such as NaI, or ScI.sub.3 also reposed within the envelope. The vapor pressures of the metal halide salts and the mercury affect both the color temperature and efficacy. These are affected in turn by the quartz envelope geometry, anode and cathode insertion depth, arc gap size, and volume of the arc chamber in the envelope. Higher operating temperatures of course produce higher metal halide vapor pressures, but can also reduce the lamp cycle by hastening quartz devitrification and causing tungsten metal loss from the electrodes. On the other hand, lower operating temperatures, especially near the bulb wall, can cause salt vapor to condense and crystallize on the walls of the envelope, causing objectionable flecks to appear in objects illuminated by the lamp.
In U.S. Pat. No. 5,083,059 of common assignee herewith, incorporated herein by reference, there is described a high efficiency metal halide discharge lamp that operates in a power range of about 5-40 watts. This lamp incorporates composite electrodes in the form of a club, having a lead-in wire of relatively small diameter, about 0.003-0.007 inches, supported in the quartz of the neck of the lamp. The lead-in wire supports a post member of greater diameter, about 0.011-0.014 inches. In this lamp the post member is supported out of contact with the quartz of the neck and also out of contact with the bulb wall. In order to minimize "dancing" of the discharge arc, the tips of the post member are conical, having taper angles of 30.degree.-45.degree. for the cathode, and 60.degree.-120.degree. for the anode.
While this lamp works well, it has been found that after prolonged operation "burn back" of the conical electrodes occurs, and the electrodes become more or less squared off. As this happens the arc gap increases, and the lamp voltage necessary to ignite and operate the lamp increases. The wattage required to operate the lamp also increases during burn back, and with some ballasts there may actually result an undesired increase in lumen output. When burn back becomes severe the operating voltage range of the lamp ballast can be exceeded, causing it to shut down.